The present invention is a process for converting chlorine end-terminated polyorganosiloxanes to polyorganocyclosiloxanes. The process comprises forming a mixture comprising chlorine end-terminated polyorganosiloxanes aqueous hydrogen chloride, and a heterogeneous reequilibrium catalyst. The mixture is heated at a temperature within a range of about 70.degree. C. to 150.degree. C. to effect reequilibrium of the chlorine end-terminated polyorganosiloxanes to form polyorganocyclosiloxanes which are removed from the process as they are formed.
When the polyorganocyclosiloxanes are removed from the process they are contaminated with chlorine end-terminated polyorganosiloxanes having similar molecular weight. Therefore, in a preferred process the mixture containing the polyorganocyclosiloxanes and chlorine end-terminated polyorganosiloxanes is refluxed to increase the molecular weight of the chlorine end-terminated polyorganosiloxanes to facilitate their recovery. The recovered chlorine end-terminated polyorganosiloxanes can be returned to the process. The described process can provide for conversion of greater than 90 weight percent of the chlorine end-terminated polyorganosiloxanes added to the process to polyorganocyclosiloxanes.
The hydrolysis of an organohalosilane, for example dimethyldichlorosilane, results in a hydrolyzate comprising a mixture of cyclic siloxanes and chlorine end-terminated short-chained polyorganosiloxanes. Much attention has been given in the art to controlling the ratio of cyclic siloxanes to polyorganosiloxane linears in the hydrolyzate. However despite the ability to control the ratio of cyclic siloxanes to linear polyorganosiloxanes in the hydrolyzate, market demand for cyclic siloxanes can exceed production capacity, or, in the process of meeting market demand for cyclic siloxanes an excess of polyorganosiloxanes linears is created.
The present process is a method for converting the chlorine end-terminated polyorganosiloxanes produced during the hydrolysis process into polyorganocyclosiloxanes. The present process is advantageous because it can use as a feed the hydrolyzate containing aqueous hydrogen chloride, chlorine end-terminated polyorganosiloxanes, and polyorganocyclosiloxanes. Unlike other processes for converting linear polyorganosiloxanes to cyclic siloxanes it is not necessary to isolate the chlorine-end terminated polyorganosiloxanes or to perform extensive washing to convert the terminal-chlorine to hydroxyl substitutions. The present process is conducted by heating a mixture comprising chlorine end-terminated polyorganosiloxanes, aqueous hydrogen chloride, and a reequilibration catalyst to effect reequilibration of the chlorine end-terminated polyorganosiloxanes to polyorganocyclosiloxanes which are removed from the reaction zone as they are formed.
The acid catalyzed reequilibration of linear polydimethylsiloxanes is known. For example, Hyde, U. S. Pat. No. 2,467,976, issued Apr. 19, 1949, describes a method for increasing the average molecular weight of a completely dehydrated polydimethylsiloxane by refluxing with hydrochloric acid. Hyde et al., U. S. Pat. No. 2,779,776, issued Jan. 29, 1957 teaches that the equilibrium reaction between a siloxane and aqueous acid is reversible and the polymer size of the siloxane at the point of equilibrium of the reversible reaction is determined by the concentration of acid in the aqueous phase.
Catalyzed, non-aqueous, systems for converting hydroxy-terminated polyorganosiloxanes to polyorganocyclosiloxanes are also known. Macher, U. S. Pat. No. 3,607,898, issued Sep. 21, 1971, describes a process where dried polymethylvinylsiloxane is heated in the presence of lithium hydroxide and a co-catalyst selected from a group consisting of alkyl polyethers and triphenylphosphine oxide. The resultant product is reported to be cyclic symtetramethyltetravinyltetrasiloxane. Lacefield, U. S. Pat. No. 3,590,064, issued Jun. 29, 1971, describes a non-aqueous process for preparing cyclic siloxanes where a halogen endblocked linear polysiloxane is reacted with at least a stoichiometric amount of an alkali metal carbonate salt in the presence of a suitable polar solvent. Kuznetsova et al., U. S. Pat. No. 3,558,681, issued Jan. 26, 1971, describes a process for making cyclic siloxanes by the thermal degradation of hydroxyl-terminated methylphenylsiloxanes contacted with lithium hydroxide or lithium silanolate.
Aqueous processes for forming polyorganocyclosiloxanes from chlorine-end terminated polyorganosiloxanes are also known. Yeboah, U. S. Pat. No. 4,423,240, describes a process where dimethyldichlorosilane is hydrolyzed in the presence of aqueous hydrochloric acid and an anionic surfactant, for example sodium lauryl sulfate, to shift the equilibrium to higher yields of polyorganocyclosiloxanes. Yeboah reports yield for the polyorganocyclosiloxanes within a range of 70 to 78 weight percent. Williams, U. S. Pat. No. 4,412,080, issued Oct. 25, 1983, reports a process for preparing polyorganocyclosiloxanes, where dimethyldichlorosilane is hydrolyzed in the presence of aqueous hydrogen chloride and a homogeneous catalyst comprising normal C6-16 alkyl sulfonic acid. This catalyst also acts as a surfactant leading to a shift of the process equilibrium to favor cyclics. The process is run as a batch process with typical yields reported to be in the range of about 63 to 89 percent cyclics. Williams, U. S. Pat. No. 4,447,630, issued May 8, 1984, describes a method of making polyorganocyclosiloxanes by hydrolyzing diorganodichlorosilanes and aqueous hydrochloric acid in the presence of a perfluorinated alkyl substituted organic material such as a perfluorinated alkyl sulfonic acid salt. In this process, the perfluorinated alkyl sulfonic acid is a homogeneous catalyst which acts as a surfactant to shift the equilibrium in favor of polyorganocyclosiloxanes. This process is reported to give yields of as high as 97 percent cyclics.
Baile et al., U. S. Pat. No. 4,689,420, issued Aug. 25, 1987, reports a process for converting polydiorganosiloxanes to polydiorganocyclosiloxanes. The process comprises (A) feeding a mixture of polydiorganosiloxanes, a catalyst, and an organic solvent to a device in which water is formed as the polydiorganosiloxanes react in the presence of the catalyst and the organic solvent, the water formed being driven out of the device as a two-phase organic solvent/water azeotrope; (B) reacting the polydiorganosiloxane-catalyst-solvent mixture from (A), essentially free of water, to convert the polydiorganosiloxanes to the desired product polydiorganocyclosiloxanes; and (C) recovering the desired product polydiorganocyclosiloxanes. Useful catalyst were reported to be alkali metal hydroxides and alkali metal silanolates. Baile et al. report that in their process the presence of water during the rearrangement reaction shifted the chemical equilibrium away from the product cyclic siloxanes in favor of linear polydiorganosiloxanes.